Imaging-optical systems as used in various applications have been the subject of substantial research and development activity directed at improving the resolving power and other optical-performance aspects of the systems. One approach to improving optical performance is simply to manufacture the system's optical elements to tighter tolerances. This approach also typically includes manufacturing and assembling the mechanical components of the system (e.g., mountings, spacers, frames, columns, and barrels) to tighter tolerances. Another approach is to control more strictly (and hence reduce variations in) the operating environment of the optical system by, e.g., controlling atmosphere, pressure, temperature, vibrations, and other parameters. These approaches have been adopted, usually in combination, with varying degrees of success, depending upon the application of the optical system. But, in certain applications, applying these approaches still does not produce satisfactory levels of optical performance from the system.
One application in which the results of applying these ameliorative approaches have been increasingly insufficient is microlithography, which is used for imprinting patterns for microcircuits, display elements, and the like on lithographic substrates (e.g., semiconductor wafers or display panels). Most microlithography now being performed utilizes deep ultraviolet (DUV) light (λ≧˜150 nm) for making exposures, and the currently emerging lithographic technology utilizes extreme ultraviolet (EUV) light (λ=11-14 nm). DUV lithography is performed using imaging optical systems that are either catadioptric (combination of refractive and reflective) or all refractive. EUV lithography is performed using imaging optical systems that are catoptric (all reflective) because no practical materials are known for making EUV lenses.
In DUV and EUV microlithography systems, and in certain other imaging-optical systems (such as astronomical telescopes), attention has been given to changing the shape of an optical element (particularly the optical surface of a reflective optical element) slightly and in a controlled manner to improve its imaging performance. Optical elements having this capability are termed “active” optics or “adaptive” optics. “Active” optics pertains to improving optical performance as otherwise affected by static factors (e.g., manufacturing tolerances) or slowly or intermittently changing factors. Example factors include: (1) change in the topology of an optical surface of an optical element due to different gravitational forces acting on the optical element (especially if the position of the optical element is changed during use); (2) change in the operating environment (e.g., temperature) of the optical element during use, causing corresponding changes in the topology of the reflective surface; (3) the optical surface having a non-ideal shape due to stack-up of manufacturing tolerances; and/or (4) the subject optical element is located upstream or downstream of other optical element(s) having their own uncorrected errors. “Adaptive” optics generally operate on a much shorter time scale than active optics to compensate for rapidly changing factors such as atmospheric effects or vibrations. For example, adaptive optics are used in certain astronomical telescopes to improve astronomical “seeing” conditions in substantially real time.
An active optical system has at least one optical element of which the geometry of an optical surface can be changed from time to time as required, at least within a defined range. The optical surface is changed by operation of passive or active actuators that deform respective regions of the optical surface as required to achieve or maintain an optimal shape of the surface. The amount of this change is normally extremely small.
Passive actuators have been used in astronomical telescopes in which the active optical system is a large mirror that is movable with respect to the earth's gravity. The actuators control and change the distribution of forces acting on the mirror to minimize distortions of the optical surface caused by a change in the direction of gravity acting on the mirror. An exemplary passive actuator for such use is a levered counterweight used in a “flotation” mounting for the mirror. The levered counterweight applies a correcting force that automatically varies in magnitude (and possibly also in direction) depending upon the direction in which gravity is acting on the counterweight. The levered counterweight can be configured to apply force to the mirror axially and/or radially. See Yoder, Opto-Mechanical Systems Design, 3rd Edition, SPIE Press, Bellingham Wash., 2006; pages 527-533. The mirror may have passive actuators only, a combination of passive and active actuators, or active actuators only. Conventional active actuators are hydraulic or pneumatic actuators. Id., pages 534-553.